Particles are added to enhance and modify the properties of many different types of compositions and products. Examples include ultra-violet (UV) light absorbing particles, pigments, colorants, fillers, matting agents, optical diffusing particles, abrasion resistant particles, viscosity modifiers, magnetic particles and reflective particles.
Particles comprising oxides are particularly suitable as additives, especially particles containing zinc oxides, titanium oxides, silicon oxides, aluminum oxides, iron oxides and/or rare-earth metal oxides. These oxides are thermodynamically stable, are typically unable to react with environmentally ubiquitous oxygen, and tend to be less reactive with water than many other oxides and non-oxide materials. These oxide materials have been used as pigments and abrasives for centuries.
Particles consisting of certain metal oxides, most notably titanium oxides, are particularly interesting because they are usually colorless and transparent to visible light, and provide protection against exposure to UV light; however, they tend to have poor photostability, caused by the photocatalytic behavior of these oxides. Metal oxides exposed to UV radiation produce an increase in free radicals. These free radicals can lead to a de-stabilization of the formulation itself. Furthermore, free radicals may cause the formation of hydroperoxides and other peroxide free radicals known to induce contact dermatitis and severe allergic reactions. These free radicals also trigger chain reactions resulting in reactive oxygen species (ROS). These highly reactive derivatives react with cellular components including lipid membranes and are considered a source of photoaging and skin cancers that appear later in life. ROS deplete and damage non-enzymatic and enzymatic antioxidant defense systems and cause permanent genetic damage. Other components in a cosmetic composition may also have low photostability, producing additional free radicals on the skin surface.
Coating particles has been found to improve the photostability of particles. Coated powders are used to prepare cosmetic compositions for application to the skin, such as compositions for protecting skin from UV radiation (for example, sunscreens).
Particles can be coated to produce coated powders having chemical stability, photostability, and the ability to form a high weight loading dispersion with low viscosity. For example, a coating was found that produced chemical stability, photostability, and such coated particles may be used to form high weight loading dispersions with low viscosity. The coating comprises (1) silica moieties, (2) organo oxysilane moieties selected from the group consisting of mono-organo oxysilane moieties, bi-organo oxysilane moieties and tri-organo oxysilane moieties, and (3) poly(dialkyl)siloxane moieties. See, for example, U.S. Pat. No. 9,139,737.
Antioxidants, often used in cosmetic skin care products, undergo oxidation processes under UV radiation, losing their effectiveness. The addition of coated powders that block and absorb UV radiation may reduce the oxidation of antioxidants caused by UV radiation. However, many UV blocking particles exhibit photocatalytic activity, such as zinc oxide and titanium oxide, and can lead to the generation of peroxide free radicals inside a cosmetic composition. This may compromise the stability and efficacy of the antioxidants, due to the prompt reaction between the antioxidants and the free radicals or the products produced by the free radicals.